In wireless communications systems, communications devices generally communicate with one another using radio transmissions that share the same transmission medium. Although such radio transmissions are normally configured to occupy allocated or assigned frequency bands, the radio-frequency spectrum is nevertheless shared by such transmissions.
Radio transmissions occupying the same parts of the shared communications spectrum can interfere with one another. The level of interference will depend on a number of factors, such as power levels of the respective transmissions and on the relative locations of the transmitters. In fact, many factors have an impact on interference.
Cognitive radios, for example, are configured to change its transmission or reception parameters to communicate efficiently without interfering with licensed users. This alteration of parameters is based on actively monitoring several factors in the external and internal radio environment, such as radio frequency spectrum, user behavior and network state.
In wireless communications systems operating with cognitive radios, the unlicensed (secondary) users can use the licensed spectrum as long as the licensed (primary) user is absent at some particular time slot and some specific geographic location. However, when the primary user reappears, ideally, the secondary users should vacate the spectrum instantly to avoid interference with the primary user.
The explosive growth in wireless services over the past several years illustrates the huge and growing demand of the business community, consumers and the government for wireless communications. With this growth of communication applications, the spectrum has become even more congested. Even though the Federal Communications Commission (FCC) has expanded some spectral bands, these frequency bands are exclusively assigned to specific users or service providers. Such expansion does not necessarily mean that the bands are being efficiently all the time.
In this regard, it has been shown that a large part of the radio frequency spectrum is vastly under-utilized. For example, cellular network bands are overloaded in most parts of the world, but amateur radio or paging frequencies are not. Moreover, those rarely used frequency bands are assigned to specific services that cannot be accessed by unlicensed users, even where transmissions of the unlicensed users will not introduce any interference to the licensed service.
To deal with the conflicts between spectrum congestion and spectrum under-utilization, cognitive radios allow secondary users to utilize licensed bands opportunistically. By detecting particular spectrum “holes” and jumping into them rapidly to meet demand for spectrum, cognitive radios can improve the spectrum utilization significantly.
To insure high spectrum efficiency while avoiding interference to licensed users, cognitive radios should be able to adapt to spectrum conditions flexibly. One approach is disclosed in U.S. Pat. No. 8,041,380 where the transmit power of a cognitive radio is controlled so that the cognitive, unlicensed radio device does not interfere with the use of a shared spectrum by a primary, licensed device. Controlling the transmit power includes determining a distance, or a function of the distance, between a primary transmitter of the primary device and the cognitive radio device based on sensing information from a spectrum sensing process. The maximum transmit power of the cognitive radio device is then dynamically controlled based on the distance, or the function of the distance, while considering a worst case scenario of an underlying cognitive radio model, to provide a quality of service requirement of the primary device.
In addition, spectrum allocation within of a cluster of communications devices is a relatively small-scale and localized process, but it can be viewed as part of a hierarchy of spectrum allocation procedures with higher-level allocation being performed at a network level and even at an inter-network level.
This is particularly the case when a number of wireless networks co-exist and they operate at the same time in adjacent or overlapping geographical areas. One approach to improve the abilities of multiple networks to co-exist and coordinate themselves is disclosed in U.S. Pat. No. 8,023,898. An interference mitigation method in a wireless communications network is provided in which a plurality of nodes share an available frequency spectrum by performing wireless communications on sub-channels defined within the available spectrum. The nodes are grouped into clusters, each cluster having a leader of the cluster. Preferably, every node in the cluster is a transceiver equipped to act as leader and this leader role is rotated around the cluster.
Even in view of the advances made for interference mitigation in wireless communications networks and systems, there is still a need to improve how multiple communications devices can operate within a shared spectrum allocation without interfering with one another.